1. Technical Field of the Invention
The present invention relates to pixel structures such as active or passive pixel sensors and pixel arrays for detecting electromagnetic radiation using, for example, MOS-based processing technology, e.g. CMOS technology. The pixel array may be used in a camera.
2. Description of Related Art
Commonly solid state image devices are implemented in a CCD-technology or in a CMOS or MOS technology.
CCD-technology is the name of a structure of adjacent MOS gates (usually NMOS devices; PMOS CCDs (Charge Coupled Devices) are not common but have been made) that allows both storage and transport of free charge. CCD imaging is performed in a three-step process: exposure of the CCD imaging device comprising an array of pixels, charge transfer, and charge-to-voltage conversion. By exposing the imaging device, an image is acquired when incident light in the form of photons falls on the array of pixels. The energy is absorbed in units called photons and a reaction takes place that creates an electron-hole pair. Exposure of the CCD imaging device thus converts light into an electronic charge at discrete sites called pixels. The charge transfer moves, at each clock pulse, the packets of charge within the silicon substrate towards an amplifier which is physically separated from the pixels. As the charge associated with one pixel moves, at the same time, the charges in all the pixels associated with that row or column move as well. For obtaining an accurate image, it is important that charges do not get lost during transfer. The packets of charge are eventually shifted to an output node, where electrons are converted to a voltage which is subsequently amplified. A disadvantage of CCD technology is that it cannot easily handle the other analog and digital functions that are critical on an imaging system level, such as analog to digital converters, precision amplifiers, memory, arithmetic units, and so on.
Of the image sensors implemented in a CMOS- or MOS-technology, image sensors with passive pixels and image sensors with active pixels are distinguished. The difference between these two types of pixel structures is that a passive pixel does not perform signal amplification whereas an active pixel does. A passive pixel sensor is simply a photodiode (MOS or p-n junction diode) with a transistor that passes photoelectricaly generated signal charge to an amplifier outside the pixel array. The term “active pixel” refers to any pixel that has an active element, that is, at least one amplifier that typically comprises one or more transistors.
Passive pixel sensors usually make serious compromises in image quality. In this type of CMOS imager, a photosensitive diode or other junction converts photons into charge. Horizontal and vertical scan registers switch the charge that has been integrated in the pixels onto readout lines, much like in a conventional semiconductor memory, and subsequently amplify it. The primary difference between a CMOS imager and a memory circuit is the amplifier precision. Disadvantages of the simple passive-pixel CMOS imager architecture are a high noise level.
Active pixel sensors have a better noise performance than their passive pixel CMOS precursors, but typically suffer from a lower fill factor (=photosensitive portion of the pixel), and thus require a larger pixel size.
However, the image quality of pixels with CMOS-technology is still less advanced than image quality of pixels with CCD-technology.
Due to the miniaturization of technology of CMOS based electronic circuits, it is possible to realize complex CMOS or MOS based pixels as small as CCD based pixels. It is a main advantage of CMOS or MOS based image sensors that CMOS technology is being offered by many foundries, whereas CCD technology is rarely offered and is a more complex and expensive technology option. Furthermore conventional CCD technology devices require significant power and a variety of input voltage levels (often in the 8–30 Volts range), while CMOS devices can be easily designed to draw only a small amount of current from a single low voltage power supply, which is important e.g. for hand-held devices.
Therefore there is an ongoing effort to increase the performance of CMOS or MOS image sensors such that a comparable image quality is obtained as the one obtained with high-end CCD images.
It is an object of the present invention to provide a pixel structure with improved charge transfer.
It is a further object of the present invention to obtain CCD-quality charge transfer using MOS-based technology.
It is yet a further object to provide an improved range pixel capable of operating at high speed.
It is also an object of the present invention to provide an array of pixels with improved properties, for instance, a synchronous shutter action.